Magnetic sensor

ABSTRACT

The present invention provides a magnetic sensor capable of reproducing a magnetic record even if the size of a recorded magnetization is a minute one, directly reading a magnetization recorded on a magneto-optical disk without applying incident light itself to the magneto-optical disk, and obtaining signals of a second harmonic having a high S/N ratio. This magnetic sensor includes a magnetic sensor element ( 102 ) having electric polarization disposed with respect to a perpendicular recording medium ( 101 ), and laser generating means acting on the magnetic sensor element ( 102 ). The magnetic sensor reads information in the perpendicular recording medium ( 101 ) based on the variation of the rotation angle φ of the polarization plane of a second harmonic ( 105 ) of a frequency 2ω exiting the magnetic sensor element ( 102 ) by the application of laser light ( 104 ) with a frequency ω from the laser generation means to the magnetic sensor element ( 102 ).

TECHNICAL FIELD

The present invention relates to a magnetic sensor. More specifically,the present invention pertains to a magnetic sensor element using asecond harmonic, and allowing spin information imbedded within a solid,such as a magneto-optical disk or hard disk device (HDD), to be read out(reproduced) with high sensitivity and high spatial resolution.

BACKGROUND ART

For the reproduction of information recorded on a magneto-optical disk,the Kerr effect of reflected light, which is a magneto-optical effect,is conventionally utilized.

FIG. 1 is a diagram explaining the reproduction principle of such aconventional magneto-optical disk.

In FIG. 1, reference numeral 1 denotes a semiconductor laser, numerals2, 4, and 5 denote lenses, numeral 3 denotes a polarizer, numeral 6denotes an analyzer, numeral 7 denotes a photodiode, numeral 8 denotesincident light, numeral 9 denotes reflected light, and numeral 10denotes a perpendicular magnetic recording film.

As shown in FIG. 1, in the reproduction principle of a magneto-opticaldisk, the polarization plane of the reflected light 9 rotates withrespect to that of the incident light 8 under the Kerr effect. Therotation angle of the polarization plane of the reflected light 9 isread and thereby storage is reproduced. Here, the rotation angleexhibits a maximum value when the direction of magnetization and thetraveling direction of light are parallel to each other. For a recordingfilm, therefore, a material having a magnetization perpendicular to thesurface of a memory medium is desirable. The conditions of having amagnetization perpendicular to the surface in this way, offers theadvantages of increasing the surface density and allowing theachievement of high density recording. Therefore, the perpendicularmagnetic recording system will become mainstream henceforward.

The memory capacity of a magneto-optical disk depends on the spot sizeof a semiconductor laser used for reproduction. The typical reproductionwavelength of the semiconductor laser is in the range of 0.78 to 0.65μm. In terms of reading accuracy, the size of magnetization is limitedto the order of reading wavelength. This results in the limitation ofrecording capacity, thus constituting the largest problem to be solvedhenceforth.

On the other hand, an invention such as a magnetically induced superresolution (MSR) system is disclosed. Use of this system is making itpossible to read out even a magnetization size of about a half of thetypical reproduction wavelength of a semiconductor laser. According toK. Shono [J. Magn. Soc. Jpn. 19, Supple, S1 (1999) 177], a recordingmark of 0.3 μm was reproduced at a wavelength of a red laser, and arecording capacity of 1.3 GB was implemented with a MO disk of 3.5 inch.However, this is also a reading size of about a half of the wavelengthat most, and therefore, it is difficult to reproduce a minutemagnetization size of not more than 0.1 μm (1000 Å). Hence, this systemhas also its limit in recording capacity as a natural result, thusconstituting a serious problem to be solved, as well.

According to conventional methods in which the magneto-optical effect isutilized for the reproduction of information, semiconductor laser lightis directly made incident onto a magneto-optical disk on which recordsare written. When the temperature increased by this incident light hasreached a temperature of not less than the spin alignment temperature(Curie temperature Tc) of the magnetic material of the magneto-opticaldisk, the storage is unfavorably erased. Accordingly, a problem occursin that the intensity of the incident light for reading must be limitedso that this transition temperature Tc or more is not reached. Thisresults in imposing restrictions on the S/N ratio improvement ofreproduction signals, thereby causing an excessively high load on areproduction signal processing system.

While the foregoing concerns problems associated with the reproductionof recording data on the magneto-optical disk, the reproduction deviceusing a magnetoresistive mechanism of a hard disk device (HDD) has alsosimilar technical problems. With the trend moving toward increasinglyminuter configurations of magnetic materials for recording, it isnecessary for the reproduction to read magnetism in ultrafine areas withhigh sensitivity.

As a next generation technology for reading HDD data, a tunnelingmagneto resistive (TMR) head (Fujikata et al., The 8th JointMMM-Intermag Conference Abstracts, p. 492, January 2001), and as atechnology of the generation after the next generation, an extraordinarymagneto resistive (EMR) head are being developed in a fierce competitionamong manufacturers.

Even in this EMR, which is referred to as a technology of the generationafter the next generation, at the stage of prototyping stage, thediameter of a reading element is several millimeters (Solin et al.,Science, vol. 289, pp. 1530–1532, September 2000), and the reading of amagnetization size of not more than 0.1 μm (1000 Å) will be a challengefor the future. The commercialization of EMR, therefore, will be a longway into the future.

DISCLOSURE OF INVENTION

Due to bloated information industries and storages of image informationand the like, demands for the increase in storage capacity seemsboundless. Therefore, with respect to the miniaturization trend inmemory size, further demands for its continuing miniaturization is beingmade. In 2004, in order to implement a memory capacity of 100 Gbpsi, thesize of magnetic material is miniaturized down to a size on the order of30 nm (300 Å). In 2007, in order to realize a memory capacity of 1000Gbpsi, the size of magnetic material is forecast to be miniaturized downto a size on the order of 10 nm (100 Å).

Accordingly, a first object of the present invention is to make itpossible to reproduce a magnetic record even if the size of a recordedmagnetization is a minute one, such as a size of several hundred orseveral ten Angstroms, or a lattice size of several Angstroms. This willdrastically increase the memory capacity of a magneto-optical disk orHDD. This method is principally different from that of the conventionalKerr rotation mechanism or magnetoresistive mechanism. This methodprovides a magnetic sensor that utilizes the rotation of thepolarization plane of a second harmonic of the reflected light withrespect to incident light, based on the nonlinear optical responsetheory associated with the asymmetry of a magnetic material.

A second object of the present invention is to provide a magnetic sensorthat does not impose limitations on the incident light intensity, thatis, a magnetic sensor that allows a magnetization recorded on amagneto-optical disk to be directly read without applying incident lightitself to the magneto-optical disk. This is for circumventing the riskof unnecessarily raising the temperature of the recording medium duringreproduction to such an extent that the recording medium is heated up toa higher temperature than the transition temperature of magnetization.

According to the magnetic sensor of the present invention, it ispossible to detect a magnetic field from the magneto-optical disk byapplying semiconductor laser light to the magnetic sensor element anddetecting a second harmonic, which is an output from the element. Inthis case, therefore, unlike the case where the conventional Kerr effectis utilized as a reproduction method, information written on themagneto-optical disk can be reproduced without directly applying lightto the magneto-optical disk.

A third object of the present invention is to provide a magnetic sensorthat, when a semiconductor laser light for generating a second harmonicis applied to a magnetic sensor element for reading a magnetizationrecorded on the magneto-optical disk, can obtain reproduction signalshaving a sufficiently high S/N ratio even if the incident lightintensity is low. According to this magnetic sensor, the rotation angleof polarization plane obtained by the magnetic sensor element is agigantic polarization rotation angle (several degrees to several tendegrees), which is about several tens to about several hundred times aslarge as the conventional rotation angle of polarization plane obtainedunder the Kerr effect. This allows the present magnetic sensor toachieve a high S/N ratio. In this connection, the rotational degree ofpolarization plane according to the conventional Kerr rotational methodis 0.3° (Tc=130° C.) for TbFe, and 0.35° (Tc=220° C.) for GdFe.

The wavelength of a second harmonic is one half as long as an incidentwavelength. Use of a wavelength filter would allow reflected wavecomponents having the same wavelength as that of the incident wave to beeasily removed, so that signals of the second harmonic with a high S/Nratio could be obtained. This also offers an advantage over the casewhere the conventional Kerr rotation is used.

As described above, the present invention aims to provide a magneticsensor capable of reproducing a magnetic record even if the size of arecorded magnetization is a minute one, directly reading a magnetizationrecorded on a magneto-optical disk without applying incident lightitself to the magneto-optical disk, and obtaining signals of a secondharmonic having a high S/N ratio.

In order to achieve the above-described objects:

-   [1] the present invention provides a magnetic sensor including a    magnetic sensor element that is disposed with respect to a body    having spin information, that has an interface structure having    spatial asymmetry, and in which one solid material constituting the    interface is a magnetic material; and laser beam irradiation means    acting on the magnetic sensor element. This magnetic sensor reads    out the spin information of the body having the spin information    based on the variation of the rotation angle of the polarization    plane of a second harmonic with a frequency 2ω exiting the magnetic    sensor element, by applying laser light with a frequency ω to the    magnetic sensor element by the laser beam irradiation means.-   [2] in the magnetic sensor set forth in the above-described [1], in    order to generate the second harmonic, at least one magnetic    material of the magnetic sensor element comprises a structure in    which the interface is constituted of a ferromagnetic material    (including a ferrimagnetic material).-   [3] in the magnetic sensor set forth in the above-described [2], in    order to generate the second harmonic, at least one material of the    magnetic sensor element is a ferromagnetic (including ferrimagnetic)    thin film material, and the other materials are multilayered thin    film materials that constitute interfaces using a plurality of thin    film materials.-   [4] in the magnetic sensor set forth in the above-described [3], out    of the plurality of thin film materials, at least one is a    transition metal film or a transition metal oxide film.-   [5] in the magnetic sensor set forth in the above-described [4], out    of the plurality of thin film materials, at least one is a manganese    oxide compound film.-   [6] in the magnetic sensor set forth in the above-described [5], out    of the plurality of thin film materials, at least one is    (A_(1-x)B_(x)) MnO₃ (0≦x≦1), wherein A is an alkaline-earth element    such as Ca, Sr or Ba, or a rare-earth element such as La, or a Y or    Bi element, and wherein B is an alkaline-earth element such as Ca,    Sr or Ba, or a rare-earth element such as La, both exclusive of A,    or an oxide comprising Y or Bi.-   [7] in the magnetic sensor set forth in the above-described [6], out    of the plurality of kinds of thin film materials, at least one kind    of thin film material is (A_(1-x)B_(x)) MnO₃ (0≦x≦1), and with a    film construction comprising the other plural kinds of films treated    as one unit, a multilayer film formed by repeating the unit a    plurality of times is used as at least one thin film material.-   [8] in the magnetic sensor set forth in the above-described [1], in    order to generate the second harmonic, at least one thin film or    crystal flake of the magnetic sensor element comprises a material    having an easy magnetization axis and a polarization axis that are    perpendicular to each other.-   [9] in the magnetic sensor set forth in the above-described [8], in    order to generate the second harmonic, one thin film material and    other plural thin film materials of the magnetic sensor element are    used, the one thin film material and the other plural thin film    materials each having the easy magnetization axis and the    polarization axis that are mutually perpendicular.-   [10] in the magnetic sensor set forth in the above-described [9], in    order to generate the second harmonic, light having an electric    field component perpendicular to the easy magnetization axis is made    incident on a material of the magnetic sensor element which material    has the easy magnetization axis and the polarization axis that are    perpendicular to each other, and the second harmonic component of    the light is used that reflects with respect to the incident light    or that is transmitted.-   [11] in the magnetic sensor set forth in the above-described [10],    out of the plurality of thin film materials of the magnetic sensor    element, at least one is a transition metal film or a transition    metal oxide film.-   [12] in the magnetic sensor set forth in the above-described [11],    out of the plurality of thin film materials of the magnetic sensor    element, at least one is a manganese oxide compound film.-   [13] in the magnetic sensor set forth in the above-described [12],    out of the plurality of thin film materials of the magnetic sensor    element, at least one is (A_(1-x)B_(x)) MnO₃ (0≦x≦1), wherein A is    an alkaline-earth element such as Ca, Sr or Ba, or a rare-earth    element such as La, or a Y or Bi element, and wherein B is an    alkaline-earth element such as Ca, Sr or Ba, or a rare-earth element    such as La, both exclusive of A, or an oxide comprising Y or Bi.-   [14] in the magnetic sensor set forth in the above-described [13],    out of the plurality of kinds of thin film materials of the magnetic    sensor element, at least one kind of film of the magnetic sensor    element is (A_(1-x)B_(x)) MnO₃ (0≦x≦1), and with a film construction    comprising the other plural kinds of films treated as one unit, a    multilayer film formed by repeating the unit a plurality of times is    used as at least one thin film material.-   [15] in the magnetic sensor set forth in the above-described [10],    [11], or [12], an iron oxide and an iron oxide thin film are used as    materials of the magnetic sensor element, the materials each having    the easy magnetization axis and the polarization axis that are    mutually perpendicular.-   [16] in the magnetic sensor set forth in the above-described [15], a    crystal and a thin film of Ga_(2-x)Fe_(x)O₃ are used, as materials    of the magnetic sensor element, the materials each having the easy    magnetization axis and the polarization axis that are mutually    perpendicular.-   [17] in the magnetic sensor set forth in the above-described [16], a    crystal and a thin film of Ga_(2-x)Fe_(x)O₃ is used that has a    rhombic crystal structure and in which x defining the composition of    iron is in the range of 0.7 to 1.5, both inclusive, as materials of    the magnetic sensor element each having the easy magnetization axis    and the polarization axis that are mutually perpendicular.-   [18] in the magnetic sensor set forth in any one of the    above-described [2] to [17], a multilayer structure is used that is    formed by sandwiching the transition metal oxide thin film of the    magnetic sensor element between SrTiO₃ thin films above and below.-   [19] in the magnetic sensor set forth in any one of the    above-described [2] to [17], a SrTiO₃ crystal is used as a material    of a substrate for supporting a plurality of thin film materials of    the magnetic sensor element.

In this invention, on the implementation of a magnetic sensor as arecord reproducing element indispensable for realizing an enormousmagneto-optical disk and HDD in a region of several terabits (Tb) psi(per square inch), the present inventors have invented a magnetic sensorusing a second harmonic in order to solve the magnetic sensor problemthat, in the conventional art, when a magnetic domain structure as aminimum unit in which information is stored has a size of not more thanabout 1000 Å, the reproduction of the record would become difficult.

The minimum constituent requirement of this magnetic sensor is that atleast one magnetic material thereof is a ferromagnetic material[including a ferrimagnetic material].

Materials other than one ferromagnetic material are not limited tosolids, but may be gases. With an interface constituted of two kinds ofmaterials being definable, the polarization plane of a second harmonicof the reflected light (transmitting light) with respect to incidentlight rotates with respect to the polarization plane of the incidentlight, and a similar effect is produced in a magnetic sensor comprisingseveral kinds of thin films. This magnetic material may be either atransition metal, or a transition metal oxide. Alternatively, themagnetic material may be one of Mn oxide compounds showing various kindsof magnetisms.

As shown in an embodiment, the magnetic material may be (A_(1-x)B_(x))MnO₃ (0.1≦x≦1), wherein A is an alkaline-earth element such as Ca, Sr orBa, or a rare-earth element such as La, or a Y or Bi element, andwherein B is an alkaline-earth element such as Ca, Sr or Ba, or arare-earth element such as La, both exclusive of A, or an oxidecomprising a Y or Bi element.

When a second harmonic is used for a magnetic sensor, a similar effectis produced even in a material having an easy magnetization axis and apolarization axis of magnetization that are perpendicular to each other.The geometrical arrangement of this magnetic sensor is set forth in thepresent invention. Magnetic (thin films) materials having polarizationand capable of constituting a magnetic sensor utilizing a secondharmonic are set forth in the present invention. A material serving assuch a multilayered protective film or substrate is set forth in thepresent invention. Their matrix is specified as an appropriate materialthat is low in misfit in lattice constants between intended thin filmmaterials and the matrix.

[Operation]

According to the present invention, it is possible to provide a magneticsensor capable of reading out spin information imbedded within a solid.This makes it possible to provide an element capable of reproducing evena minute storage area of less than 1000 Å, such as several hundredangstroms or several angstroms as an reproduction element of amagneto-optical disk. This element can also be put into practical use asa reading device for a hard disk device (HDD). Use of this reproductionelement allows the storage capacity to be raised up to a region of 1000Gbpsi in a stroke.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the reproduction principle of aconventional magneto-optical disk.

FIG. 2 is a principle view of a magnetic sensor using a second harmonicaccording to the present invention.

FIG. 3 is a sectional view showing the structure of a superlattice LSMOmagnetic sensor element according to the present invention.

FIG. 4 is a diagram showing the disposition relationship between thesuperlattice LSMO magnetic sensor element according to the presentinvention and laser light.

FIG. 5 is a graph showing rotational characteristics of the polarizationplane of a second harmonic on the superlattice LSMO magnetic sensorelement according to the present invention.

FIGS. 6( a) to 6(d) are diagrams showing the incident angle dependenceof the polarization plane of a second harmonic on the superlattice LSMOmagnetic sensor element according to the present invention.

FIGS. 7( a) to 7(d) are diagrams showing the temperature andmagnetization dependence of the polarization plane of a second harmonicon the superlattice LSMO magnetic sensor element according to thepresent invention.

FIG. 8 is a diagram showing the disposition relationship between thecrystal structure of a GaFeO₃ magnetic sensor element according to thepresent invention and laser light.

FIGS. 9( a) to 9(d) are diagrams showing the temperature andmagnetization dependence of the polarization plane of a second harmonicon the GaFeO₃ magnetic sensor element according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments according to the present invention will bedescribed with reference to the drawings.

FIG. 2 is a principle view of a magnetic sensor using a second harmonicaccording to the present invention.

In FIG. 2, reference numeral 101 denotes a perpendicular magneticrecording film (an object having spin information, i.e., a recordingmedium), numeral 102 denotes a sensor element, numeral 103 denotes amagnetic material, numeral 104 denotes laser light with a frequency ω,and numeral 105 denotes a second harmonic with a frequency 2ω.

Suppose the laser light 104 is made incident on the sensor element 102at an incident angle α. Here, the material constituting the sensorelement 102 is assumed to be the magnetic material 103. When themagnetic material 103 feels a positive or negative magnetic field(magnetic flux density=B) generated by downward magnetization (data: 0)or upward magnetization (data: 1) of the perpendicular magneticrecording film 101, the direction of the spin of a magnetic domainbecomes the downward direction (x-direction for the ferromagneticmaterial 103) or the upward direction. The magnetic material 103generates the second harmonic 105 with a frequency 2ω, and thepolarization plane of the second harmonic 105 rotates by a degree of +φor −φ. An observation of the polarization plane allows the direction ofspin of the perpendicular magnetic recording film 101 to be detectable.The magnetic material 103, therefore, performs the function of themagnetic sensor element 102.

Pustogowa [Phys. Rev. B49 (1994) 10031] et al. theoretically predicted,using an iron metal thin film, that the polarization plane of a secondharmonic from the interface of a magnetic material rotates, and Rasing[Phys. Rev. Lett. 74 (1995) 3692, J. Appl. Phys. 79 (1996) 6181] et alobserved the rotation of the polarization plane of a second harmonic, ona Fe/Cr film formed of a sputter film.

The rotation angle then was 34°, which is orders of magnitude largerthan the conventional Kerr's rotational angle (0.03°). This principlemakes use of the generation of a second harmonic as a result ofintroducing an asymmetry of a material due to the formation of aninterface.

The present invention constructed a magnetic sensor with attentionfocused to the above-described principle. Specifically, because of itssensitiveness to an interface, this principle could be utilized for amonitor for spin states. Furthermore, by taking advantage of the factthat the direction of spin reacts to a magnetic field generated by amagnetization constituting a memory, the present invention has succeededin incorporating this principle in a magnetic sensor.

In essence, the magnetic sensor using a second harmonic has only to beformed into a structure having an asymmetry. Therefore, in principle, itis essential only that an interface with a size of several angstroms canbe formed. This is because the interface with a size of severalangstroms becomes a limiting factor in the spatial resolution of themagnetic sensor, thereby allowing a magnetization in a several angstromregion to be read. Although, in actuality, depending on the flatness ofan interface and on the film thickness of a thin film as shown inembodiments, the spatial resolution of the magnetic sensor enables datareading up to just the ultimate minute region.

Moreover, in the present invention, it is demonstrated that theabove-described effect is enhanced when a material constituting such anelement has electric polarization P. In FIG. 2, with the polarization Pfaced in a z-axis, and with the direction of magnetization being x-axisdirection, when the electric field vector E of incident light has ay-direction component, a second harmonic occurs. A second ordersusceptibility χ(2) is expressed byχ_(yyy)(2)=αMxPzThat is, the existence of Pz produces χ_(yyy)(2). This indicates thatthe material having polarization can enhance the above-described effect.The rotation angle of polarization plane is in proportion to themagnitude of a generated magnetization. Within a weak magnetic field,the magnitude of magnetization is substantially in proportion to anexternal magnetic field, and therefore, it depends on the magnetic fieldstrength in which the magnetization of a memory occurs.

Therefore, the larger the obtained rotation angle of a polarizationplane, the smaller the detectable lower limit of magnetic field strengthcan be made. With the perpendicular magnetization as shown in FIG. 2,the direction of spin reverses between recording data 0 and 1. In otherwords, the angle of polarization plane rotates in the positive andnegative directions. This also allows a signal indicating the directionof magnetization to be easily read. The leakage magnetic field from amagnetic domain of an actual magnetic memory is several oersteds. Verylarge rotations of polarization planes shown in embodiments allow thisminute leakage magnetic field to be sufficiently detectable.

FIRST EMBODIMENT

An concrete example having a superlattice structure and used as amagnetic sensor element is shown below.

FIG. 3 is a sectional view showing the structure of an LSMO magneticsensor element with a superlattice structure according to the presentinvention.

This superlattice employed the (100) plane of SrTiO₃ (abbreviated asSTO) 201 as a substrate. On this substrate (100) plane, ten molecularlayers of La_(0.6)Sr_(0.4)MnO₃ (abbreviated as LSMO) 202 were stacked.On these LSMO layers, two molecular layers of LaAlO₃ (abbreviated asLAO) 203 and three molecular layers of STO 204 were stacked. The filmthickness in a single layer is 3.824 Å for LSMO 202, 3.750 Å for LAO203, and 3.905 Å for STO 204. With the structure constituted of thesemolecular layers of LSMO 202, LAO 203, and STO 204 were treated as oneunit, ten units thereof were stacked on the above-described structure.The film thickness of the one unit is 57.45 Å, that of the ten unitsbeing 574.55 Å. The stacking of these molecular layers were performedusing a laser abrasion method. The number of stacked layers wasdetermined by an observation using a reflection high-energy electrondiffraction (RHEED) method. The purpose of using STO 201 as thesubstrate is to reduce misfit in lattice constants between the substrateand the films stacked thereon. Here, LSMO 202 is a ferromagnetic filmand has an easy magnetization axis within the surface thereof.

FIG. 4 is a diagram showing the disposition relationship between thesuperlattice LSMO magnetic sensor element according to the presentinvention and laser light.

In FIG. 4, reference numeral 301 denotes a magnetic film disposed sothat the upper interface structure and lower interface structure of LSMOare different from each other, that is, a magnetic sensor element with asuperlattice structure. Reference numeral 302 denotes incident laserlight with a frequency ω, and numeral 303 denotes a second harmonic witha frequency 2ω.

As the incident laser light 302, an S-wave polarized light is used thathas an electric field vector existing within the surface of the magneticsensor element. Here, the incident angle is assumed to be α. The secondharmonic 303 then generated becomes p-polarized light when an asymmetryof interface exists. The superlattice-structured magnetic sensor elementshown in FIG. 3 has an asymmetric structure. Specifically, with one kindof magnetic film group LSMO 202 noted, a film group thereabove is LAO203, and film group therebelow is STO 204. In this case, the upperinterface structure and lower interface structure of LSMO 202 arearranged to differ from each other.

In the event that the film group above LSMO is STO 204 on the contrary,the second harmonic 303 generating from the upper interface and thatgenerating from the lower interface would have mutually opposite phasesand cancel each other out, so that the second harmonic 303 would notoccur. That is, in an interface where symmetry is retained, no secondharmonic occur. Since the structure shown in this embodiment has anasymmetry, it generates the second harmonic 303 of p-polarized light.

Furthermore, when magnetization occurs in LSMO 202, the generated secondharmonic 303 becomes s-polarized light. As a result, in the structurecomprising the superlattice material in the illustrated embodiment, thepolarization plane of the second harmonic 303 provides a composite waveof a p-wave due to the asymmetry and an s-wave due to the magnetization.The purpose of stacking ten of basic units is to enhance the intensitysignal of the second harmonic 303. It is supposed that, if the units ofthe stacked films is transparent with respect to the energy of theincident light 302, the signal intensity of the second harmonic 303 willalso be high. In this embodiment, as an incident energy, 1.55 eV wasselected. Because the s polarization intensity of the second harmonic303 is in proportion to the intensity of magnetization, the rotationalangle of polarized light depends on the intensity of magnetization.

FIG. 5 is a graph showing rotational characteristics of the polarizationplane of a second harmonic on the superlattice LSMO magnetic sensorelement. Here, symbols ● and ◯, respectively, indicate cases of +0.35 Tand −0.35 T.

In our experiments, magnetization was generated by applying an externalmagnetic field of 0.35 Tesla (0.35 T) along each of positive andnegative directions, and polarization angles of the second harmonic waremeasured.

FIG. 5 shows measured examples of polarization angles and secondharmonic generation (SHG) intensities under the conditions that anincident energy=1.55 eV, incident angle=13°, sample temperature=10K.When (a) B=+0.35 T and (b) B=−0.35 T, the polarization-angle relativerotational angle 2φ between the cases (a) and (b) exhibited 33°, whichis a large relative rotational angle as might be expected. For amagnetic sensor element, it is essential only that an analyzer beprovided on the side of output light of SHG and that the intensity oflight exiting this analyzer be detected. This is the same thing asreading out stored “up spin” and “down spin” along the magnetizationaxis by the magneto-optical Kerr rotation shown in FIG. 1.

In actuality, the analyzer has a polarization plane inclined at a degreeof 45°, and therefore, as shown in each of FIGS. 6( c) and 6(d), thedifference in signal intensity at 45° is to be read, that is, the SHGintensity difference between +B(45°) and −B(45°) is to be read. This SHGintensity difference exhibits a signal intensity variation ofapproximately 50% with respect to the maximum value of SHG signalintensity. Therefore, the rotation of polarization angle of 33°generates as a memory reproduction signal that is a very large signalvariation, thus providing a very large reproduction signal.

FIGS. 6( a) to 6(d) are diagrams showing the incident angle dependenceof the polarization plane of a second harmonic on the superlattice LSMOmagnetic sensor element according to the present invention. FIG. 6( a)shows the polarization plane of a second harmonic when an incident angleα₁ of laser light with a frequency ω is 26°, and FIG. 6( b) shows thepolarization plane of a second harmonic when an incident angle α₂ oflaser light with a frequency ω is 13°. FIG. 6( c) is a characteristicview showing the relationship between the rotation angle φ ofpolarization plane and the SH intensity corresponding to FIG. 6( a), andFIG. 6( d) is a characteristic view showing the relationship between therotation angle φ of polarization plane and the SH intensitycorresponding to FIG. 6( b). In FIG. 6( a), laser light 401 with afrequency ω is made incident, and a second harmonic 402 with a frequency2ω is obtained. On the other hand, in FIG. 6( b), laser light 501 with afrequency ω is made incident, and a second harmonic 502 with a frequency2ω is obtained.

The rotation angle φ of polarization plane depends on the incident angleα (see the inset in FIG. 6( c)). The smaller the incident angle α, thelarger the rotation φ of polarization plane. Therefore, a smallerincident angle α is advantageous, but it involves devising an opticallayout. Unlike the case of common magnetization detection by a Kerrrotation (Kerr effect), the energy of exiting light is twice as high asthat of incident light, and therefore, use of an appropriate opticalfilter allows the separation of the exiting light from the incidentlight.

The temperature dependence of the rotation of polarizing plane with theincident angle α₁ being 26°, reflects the temperature dependence of themagnetization of LSMO. The rotation angle φ increases with an increasein magnetization.

FIGS. 7( a) to 7(d) illustrate the temperature and magnetizationdependence of the polarization plane of second harmonic on thesuperlattice LSMO magnetic sensor element according to the presentinvention. FIG. 7( a) shows the case where temperature is 10K, with thedifference in rotation angle between the cases of +0.35 T (●) and −0.35T (◯) being ±7.9°. FIG. 7( b) shows the case where temperature is 100K,with the difference in rotation angle between the cases of +0.35 T (●)and −0.35 T (◯) being ±3.6°. FIG. 7( c) shows the case where temperatureis 300K, with the difference in rotation angle between the cases of+0.35 T (●) and −0.35 T (◯) being ±0.2°. FIG. 7( d) is a characteristicview showing the relationship between the magnetization (μ_(B)/Mn) andthe temperature (K). It can be seen that the rotation angle φ of thepolarization plane of second harmonic increases with an increase inmagnetization.

According to the obtained results for the superlattice LSMO, with anincident angle α being 13°, the polarization plane of second harmonicrotated by 33° in the positive and negative directions of a magneticfield. In the present embodiment, this rotation angle was obtained under0.35 T=3500 Oe. In this way, relatively strong external magnetic fieldwas used due to limitations on an experimental apparatus, but it can beseen from the magnetization data shown in FIG. 7( d) that thissuperlattice material exhibits the same magnetization intensity in anexternal magnetic field as in the case of 100 Oe. In other words, evenin the external magnetic field of 100 Oe, a rotation angle on the orderof 33° can be obtained. Current magnetic optical systems for readingKerr rotation industrially have a sufficient accuracy to read a rotationon the order of 0.1°. If a leakage magnetic field from the perpendicularmagnetization film is 1 Oe, the rotation angle of the polarization planeof second harmonic will be on the order of 0.3°. This indicates thatcurrent magnetic optical systems have an industrially sufficientidentification accuracy.

The film thickness of this superlattice is 575 Å as shown in FIG. 3, andtherefore, as the minimum detectable magnetization size, the size on theorder of 575 Å is feasible. In actuality, because signal processingtechniques are advanced, the size on the order of 300 Å, which is abouta half of 575 Å, is also feasible. Thus, even a magnetization size of afew hundred angstroms can be detected. The detection capability withrespect to a minute size can be further improved by reducing the numberof the above-described units. Since, in principle, even a single layercan be detected, it might be possible to detect even the film thickness3.82 Å of a single layer of LSMO. Thus, the detection capability withrespect to a minute size on just the angstrom order might be secured.

The temperature shown in this embodiment is 10 K, but the magneticsensor using a second harmonic is sufficiently operable even at roomtemperature, by using a superlattice structure that employs a materialhaving a sufficiently high ferromagnetic transition temperature, as amagnetic material.

SECOND EMBODIMENT

A concrete example is shown in which a rhombic crystal GaFeO₃ isincorporated in a magnetic sensor.

This crystal has a rhombic crystal structure, and it has a space groupof Pc2_(1n) according to the classification table of space groups. Thiscrystal has a structure such that the easy magnetization axis thereofconstitutes a c-axis, and the polarization axis thereof constitutes ab-axis. The dispositions of this crystal structure, the polarizationdirection of incident light, and an SHG signal are shown in FIG. 8.

FIG. 8 is a diagram showing the disposition relationship between thecrystal structure of the GaFeO₃ magnetic sensor element according to thepresent invention and laser light. Here, the polarization axis, i.e.,b-axis, of GaFeO₃ is arranged to be a z-axis, and the easy magnetizationaxis thereof is arranged to be an x-axis. Laser light is made incidentparallel to the b-axis, i.e., along the z-axis direction.

In FIG. 8, reference numeral 601 denotes a magnetic sensor elementcomprising a GaFeO₃ rhombic crystal, numeral 602 denotes laser lightwith a frequency ω, and numeral 603 denotes a second harmonic with afrequency 2ω.

GaFeO₃ has a ferrimagnetic property, and its transition temperature is210 K. The temperature dependence of the magnetization M of GaFeO₃ isshown with a solid line in FIG. 9( d).

FIGS. 9( a) to 9(c) show measurement results regarding SHG intensitieson the GaFeO₃ magnetic sensor element and the polarization plane underthe conditions: an incident energy=1.55 eV, an s-polarized incidentlight angle=26°, with temperature being 250 K (FIG. 9( c)), which isabove the ferrimagnetic transition temperature Tc (210 K), 180 K (FIG.9( b)), which is directly below Tc, and 100 k (FIG. 9( a)), which issufficiently lower than Tc.

Here, the magnetic field is applied in parallel with the c-axis, and themagnetic field strength is set to B=+0.35 T and B=−0.35 T because ofrestrictions on the experimental arrangement.

The rotation angles φ of polarization plane were φ=0 at 250 K, φ=±45° at180 K, and φ=±80° at 100 K. Thus, the rotation angles φ exhibitedgigantic values. The relationship between the rotation angle and thetemperature is shown using a symbol ◯ in FIG. 9 (d). As can be seen fromFIG. 9( d), this temperature dependence of the rotation angle conformsto the tendency of the magnetization curve of GaFeO₃. In other words,regarding the temperature dependence, the rotation angle of polarizationis in conformance with the magnetic susceptibility of this magneticsensor. The polarization rotating angles of SHG at 100 K were ±80°(±0.35 T), that is, a polarization angle relative rotation angle2φ=160°, which is a gigantic polarization rotation angle, was obtained.As shown in the first embodiment, in actuality, with a polarizerprovided at an angle of 45° on the SHG signal side, the direction ofmagnetic field is to be read, and therefore the polarization rotationangle is too large at 100 K. With a temperature on the order of 180 Kprovided, the output intensity of the polarizer could vary up to themaximum intensity 100% of SHG. The actual polarization rotation angledepends on the magnetic field strength generated by the magnetization ofa memory, and the rotation angle becomes smaller. Therefore, the higherthe capability of rotating responsively to the magnetic field, thebetter.

The GaFeO₃ crystal is produced by a floating zone melting method(Japanese Patent Application No. 2002-234708).

Specifically, a single crystal Ga_(2-x)Fe_(x)O₃ having a rhombic crystalstructure is produced by a floating zone melting method in which thefront ends of sample bars formed of Ga_(2-x)Fe_(x)O₃ and disposed on theupper and lower sides, is heated using a heat source placed at aco-focus position in a gas atmosphere, and in which a floating zone isformed between the front ends of the above described sample bars formedof Ga_(2-x)Fe_(x)O₃.

Ga_(2-x)Fe_(x)O₃ has a Tc of 210 K when x=1. When x=1.4,Ga_(2-x)Fe_(x)O₃ has a Tc of 360 K and hence it has the above-describedeffect at room temperature. With x=1.4, therefore, Ga_(2-x)Fe_(x)O₃ canbe put to service as a magnetic sensor element to be used at roomtemperature.

The magnetic sensor element according to this embodiment is formed of asingle crystal flake. Currently, one means for taking out a singlecrystal flake is a field ion beam (FIB) apparatus. Use of this apparatusmakes it possible to take out a crystal flake of 100 μm square andhaving a film thickness on the order of several hundred Angstroms and toset the crystal flake in a state of being oriented. As a consequence,the detection with respect to a minute magnetization size on the orderof several hundred Angstroms becomes feasible.

The present invention is not limited to the above-described embodiments,but it is to be understood that various modifications and variations maybe made within the true spirit of the present invention, and that thesemodifications and variations are not excluded from the scope of thepresent invention.

As described above in detail, according to the present invention, it ispossible to provide a magnetic sensor element capable of detecting aminute magnetic domain structure in a several hundred Angstrom region bymeans of a system different from the conventional ones. This element iscapable of detecting a minute magnetic domain structure even in severalten Angstrom region, which leads to the solution of a large problemassociated with data reproduction devices in magnetic recordingapparatuses. This allows an enormous magnetic memory device in terabitregion to be provided, thereby enabling an enormous memory suitable forinformation communications and an optical computer to be provided.

Also, this sensor element is not limited to application to areproduction device for a magnetic memory. For example, when suppliedwith a current, a coil generates a magnetic field. By using thisprinciple, the polarization plane of second harmonic can be easilycontrolled. Specifically, by providing a polarizer on the output side,light can be easily turned on/off. This makes it possible to apply thissensor element to a current controlled optical switching element in alight communication network, and to provide this sensor element with afunction as a light modulation element.

Furthermore, since the magnetic sensor can sensitively detect a magneticfield in a minute region, for example, by disposing a minute magnet onone side and mounting the magnetic sensor proposed herein on the otherside, the present invention can also be applied to an opening/closingsensor (e.g., an opening/closing sensor for a mobile phone). In thismanner, the present invention is not restricted to application to amagnetic memory, but it would also be adaptable for use as a basicelement in a wide-ranging information network.

INDUSTRIAL APPLICABILITY

The magnetic sensor according to the present invention has a highsensitivity and high spatial resolution, and hence, it is particularlysuitable for a reproduction device for a magnetic memory. Moreover, thismagnetic sensor is adaptable for use as a basic device associated withoptical communications.

1. A magnetic sensor comprising: (a) a magnetic sensor element that isdisposed with respect to a body having spin information, that has aninterface structure having spatial asymmetry, and in which one solidmaterial constituting the interface is a magnetic material; (b) laserbeam irradiation means acting on the magnetic sensor element; and (c)the magnetic sensor reading out the spin information of the body havingthe spin information based on a variation of a rotation angle of apolarization plane of a second harmonic with a frequency 2ω exiting themagnetic sensor element, by applying laser light with a frequency ω tothe magnetic sensor element by the laser beam irradiation means.
 2. Themagnetic sensor according to claim 1, wherein, in order to generate thesecond harmonic, at least one magnetic material of the magnetic sensorelement comprises a structure in which the interface is constituted of aferromagnetic material including a ferrimagnetic material.
 3. Themagnetic sensor according to claim 2, wherein, in order to generate thesecond harmonic, at least one material of the magnetic sensor element isa ferromagnetic including ferrimagnetic thin film material, and whereinthe other materials are multilayered thin film materials that constituteinterfaces using a plurality of thin film materials.
 4. The magneticsensor according to claim 3, wherein, out of the plurality of thin filmmaterials, at least one is a transition metal film or a transition metaloxide film.
 5. The magnetic sensor according to claim 4, wherein, out ofthe plurality of thin film materials, at least one is a manganese oxidecompound film.
 6. The magnetic sensor according to claim 5, wherein, outof the plurality of thin film materials, at least one is (A_(1-x)B_(x))MnO₃ (0≦x≦1), wherein A is an alkaline-earth element or a rare-earthelement, and wherein B is an alkaline-earth element or a rare-earthelement both exclusive of A, or an oxide.
 7. The magnetic sensoraccording to claim 6, wherein, out of the plurality of kinds of thinfilm materials, at least one kind of thin film material is(A_(1-x)B_(x)) MnO₃ (0≦x≦1), and wherein, with a film constructioncomprising the other plural kinds of films treated as one unit, amultilayer film formed by repeating the unit a plurality of times isused as at least one thin film material.
 8. The magnetic sensoraccording to claim 2, wherein a multilayer structure is used that isformed by sandwiching the transition metal oxide thin film of themagnetic sensor element between SrTiO₃ thin films above and below. 9.The magnetic sensor according to claim 2, wherein a SrTiO₃ crystal isused as a material of a substrate for supporting a plurality of thinfilm materials of the magnetic sensor element.
 10. The magnetic sensoraccording to claim 1, wherein, in order to generate the second harmonic,at least one thin film or crystal flake of the magnetic sensor elementcomprises a material having an easy magnetization axis and apolarization axis that are perpendicular to each other.
 11. The magneticsensor according to claim 10, wherein, in order to generate the secondharmonic, one thin film material and other plural thin film materials ofthe magnetic sensor element are used, the one thin film material and theother plural thin film materials each having the easy magnetization axisand the polarization axis that are mutually perpendicular.
 12. Themagnetic sensor according to claim 11, wherein, in order to generate thesecond harmonic, light having an electric field component perpendicularto the easy magnetization axis is made incident on a material of themagnetic sensor element, the material having the easy magnetization axisand the polarization axis that are perpendicular to each other, andwherein the second harmonic component of the light is used that reflectswith respect to the incident light or that is transmitted.
 13. Themagnetic sensor according to claim 12, wherein, out of the plurality ofthin film materials of the magnetic sensor element, at least one is atransition metal film or a transition metal oxide film.
 14. The magneticsensor according to claim 13, wherein, out of the plurality of thin filmmaterials of the magnetic sensor element, at least one is a manganeseoxide compound film.
 15. The magnetic sensor according to claim 14,wherein, out of the plurality of thin film materials of the magneticsensor element, at least one is (A_(1-x)B_(x)) MnO₃ (0≦x≦1), wherein Ais an alkaline-earth element or a rare-earth element, and wherein B isan alkaline-earth element or a rare-earth element, both exclusive of A,or an oxide.
 16. The magnetic sensor according to claim 15, wherein, outof the plurality of kinds of thin film materials of the magnetic sensorelement, at least one kind of film material is (A_(1-x)B_(x)) MnO₃(0≦x≦1), and wherein, with a film construction comprising the otherplural kinds of films treated as one unit, a multilayer film formed byrepeating the unit a plurality of times is used as at least one thinfilm material.
 17. The magnetic sensor according to claim 12, wherein aniron oxide and an iron oxide thin film are used as materials of themagnetic sensor element, the materials each having the easymagnetization axis and the polarization axis of the magnetic sensor thatare mutually perpendicular.
 18. The magnetic sensor according to claim17, wherein a crystal and a thin film of Ga_(2-x)Fe_(x)O₃ are used, asmaterials of the magnetic sensor element, the materials each having theeasy magnetization axis and the polarization axis that are mutuallyperpendicular.
 19. The magnetic sensor according to claim 18, wherein, acrystal and a thin film of Ga_(2-x)Fe_(x)O₃ is used that has a rhombiccrystal structure and in which x defining the composition of iron is inthe range of 0.7 to 1.5, both inclusive, as materials of the magneticsensor element each having the easy magnetization axis and thepolarization axis that are mutually perpendicular.